1. Field of the Invention
This invention relates to a method and apparatus for electrophotography utilizing a photosensitive screen having therein a large number of tiny openings. More particularly, the present invention is concerned with a method and apparatus for forming an original image to be reproduced by modulating ion current over a repeated number of times through one and the same primary electrostatic latent image formed on such photosensitive screen.
2. Description of the Prior Arts
For representative electrophotographic tecniques which have so far been known, there can be exemplified a direct method such as electro-fax method, and an indirect method such as xerographic method.
In the formation of an image by the former method, i.e., direct method, there has been used an image recording member produced by coating a substrate with a photoconductive substance such as, for example, zinc, oxide, etc., and by some other special treatment. On account of this, the image formed on the finished image recording material lacks brightness, hence a problem of insufficient image contrast has always been experienced by those who use such recording member. Furthermore, the recording member thus treated has a defect such that it possesses senses of touch and weight which are somewhat different from those of plain paper for general use.
In the formation of an image by the latter method, i.e., indirect method, the desired image can be formed by the use of plain paper as the recording member, hence the image of good contrast can be advantageously obtained. On the other hand, however, the method is not free from any defect in that, when the toner image is to be transferred onto the recording member, it inevitably contacts the surface of the photosensitive body, and, moreover, when the residual toner is to be cleaned, the cleaning device such as brush, elastic body, and so forth vigorously rubs this photosensitive body to possibly impair its surface. Such unfavorable phenomena in the image recording operations would severely curtail service life of the expensive photosensitive body, which would result in inviting increased cost in the image reproduction.
As an expedient to remove various problems which have so far been experienced in the heretofore known electrophotographic technique, there can be exemplified the electrophotographic method as taught in U.S. Pat. No. 3,713,734. In this patented method, a photosensitive screen or lattice structure having therein numerous tiny openings in the form of a network is utilized. With this screen or lattice, an electrostatic latent image is formed on a recording member by modulating ion current through this screen, etc.; after which the electrostatic latent image on this recording member is developed for visual image. That is, in this electrophotographic method, there is no necessity for development and cleaning of the photosensitive screen or lattice which corresponds to the photosensitive body in the conventional apparatus. Therefore, the electrophotographic method as taught in this patent possesses remarkable advantages as compared with the conventional photosensitive body, such that the photosensitive screen, etc, can be used over a long period of time.
Also, which modulate the ion current by use of the photosensitive screen or lattice as mentioned above, there is such one that enables the modulation of the ion current to be carried out for a number of repeated times with one and the same primary electrostatic latent image formed on the screen, etc. In case of producing a number of sheets of reproduced images from one and the same original by using such photosensitive screen, i.e., when the so-called "retention copying" is to be performed, the image forming speed can be remarkably improved for the undermentioned reasons.
That is to say, in modulating the ion current after the second cycle of the image reproduction, the time required for forming the primary electrostatic latent image on the screen can be dispensed with. This adds a remarkable effect to the retention copying where, in particular, the process for forming the electrostatic latent image on the screen is very complicated and longer time is necessary for that, or the image forming speed of the primary electrostatic latent image is very slow due to the responding speed to light of the photosensitive screen being slow on account of the characteristic of the photoconductive member used therefor. Generally speaking, however, when the ion current modulation is continuously carried out on the screen for a plurality of times with one and the same primary electrostatic latent image, there is inevitably brought about a large difference in the electric potential between the secondary electrostatic latent image formed on the recording member at the initial stage and that formed at the last stage, as the number of times for the modulation becomes increased. In other words, the latent image at the initial stage has a high electrostatic contrast and is able to produce favorable images when developed, while the latent image at the last stage of retention copying has its electrostatic contrast lowered to a certain extent, and, when developed, latent image there arises undesirable results such that the image density is considerably lowered, or the fogging phenomenon takes place in the reproduced image. Also, depending on the kind of screen, there takes place not only the potential difference in the secondary electrostatic latent image between the initial and last ones as mentioned above, but also remarkably potential difference between the first and second ones, or between the first and second or a few subsequent ones. On account of this, even the retention copying technique which has made it possible to theoretically shorten the image forming time still possesses various actual problems and defects to be solved in respect of nonuniformity in the modulated ion current. Solution of these problems and defects can only lead to practicability in the retention copying.